Note that some people prefer using "sentence case" in titles, where you only capitalize words as you would in a normal sentence. This is also known as "Down style." Apparently this is more typical in the UK than the USA, but opinions vary (as you might expect) on which is the correct form, or if there is even one.

It is also often up to the authors/editors discretion, for example if your title was fairly long (such as an essay) it might look very strange if a couple of words are lowercase. Also, of course, it should be consistent throughout, a lot of people seem to forget that!